1. Field of the Invention
The present invention relates to a angular velocity detector, and more specifically an apparatus which detects a rotational angular velocity utilizing a ring laser type gyroscope.
2. Related Background Art
Conventionally known as gyroscopes for detecting angular velocities of moving bodies are mechanical gyroscopes having rotors and oscillators as well optical gyroscopes. The optical gyroscopes in particular which can be started momentarily and have broad dynamic ranges are renovating the art of gyroscope technology. The optical gyroscopes are classified into a ring laser type gyroscope, an optical fiber gyroscope, a passive type ring resonator gyroscope and so on. Out of these optical gyroscopes, the ring laser type gyroscope which uses a gas laser was developed earliest and has already been put to practical used in aircrafts and so on. As a ring laser type gyroscope which is compact and has a high accuracy, there has recently been available a semiconductor laser gyroscope which is integrated on a semiconductor substrate.
FIG. 12 is a plan view illustrating an optical gyroscope which is capable of detecting not only an angular velocity but also a rotational direction. Reference numeral 10 denotes a quartz tube, reference numeral 11 denotes an asymmetrical tapered portion of a light waveguide path, reference numeral 12 denotes a mirror, reference numeral 13 denotes an anode, reference numeral 14 denotes an electrode terminal, reference numeral 15 denotes a cathode, reference numeral 100 denotes a counterclockwise laser beam and reference numeral 110 denotes a clockwise laser beam.
In a configuration described above, the quartz tube 10 is formed by boring a quartz block. Then, the mirror 12 is attached to the quartz tube 10. Furthermore, the anode 13, the electrode terminal 14 and the cathode 15 are attached to the quartz tube 10. Then, the quartz tube 10 is filled with helium gas and neon gas, and a voltage is applied across the anode and the cathode to start electric discharge and supply an electric current. The counterclockwise laser beam 100 and the clockwise laser beam 110 are oscillated in the quartz tube 10.
When the quartz tube 10 stands still, the laser beam 100 and the laser beam 110 have substantially the same oscillation frequency of 4.73xc3x971015 Hz and an oscillation wavelength xcex of 632.8 nm. However, an oscillation threshold value for the laser beam 100 is smaller than that for the laser beam 110 since the tapered portion 11 of the light waveguide of the path has an asymmetrical shape. Accordingly, the laser beam 100 has an optical intensity which is higher than that of the laser beam 110. As a result, an oscillation frequency f1 of the laser beam 100 is 20 MHz higher than an oscillation frequency f2 of the laser beam 110. The laser beam 100 and the laser beam 110 interfere with each other in the quartz tube 10. A signal having an amplitude of 100 mV and a frequency of 20 MHz is obtained by adjusting a power source current so as to be constant and monitoring a voltage across the electrode terminal 14 and the cathode 15. In other words, a beat voltage can be detected even when the quartz tube 10 stands still.
When the quartz tube 10 is rotated clockwise at a velocity of 1 degree per second and a side of the resonator is 10 cm long, the oscillation frequency f1 of the counterclockwise laser beam 100 is increased by 248.3 kHz. On the other hand, the oscillation frequency f2 of the clockwise laser beam 110 is decreased by 248.3 kHz. Accordingly, the beat frequency is (f1xe2x88x92f2)=20 MHz+496.6 kHz. When the quartz tube 10 is rotated counterclockwise at a velocity of 1 degree per second, on the other hand, the beat frequency is (f1xe2x88x92f2)=20 MHzxe2x88x92496.6 MHz. Since absolute values of the increase and the decrease of the beat frequency are proportional to the rotational speeds and the rotational directions have one-to-one correspondence to the increase and decrease of the beat frequency, the optical gyroscope is capable of detecting not only rotational speeds but also rotational directions. Though the quartz tube 10 is driven with the constant current and changes of a terminal voltage are measured in this example, changes of a current supplied to a terminal may be detected when the quartz tube 10 is driven with a constant voltage. Furthermore, changes of impedance of electric discharge may be detected directly with an impedance meter. Though helium gas and neon gas are used in this example, any gas is usable so far as the gas can be excited by a laser beam. Furthermore, the light waveguide path may have any form which surrounds a rectangle, a hexagon, a triangle, a circle or the like.
The gyroscope indicates changes of a terminal voltage having a predetermined frequency due to interference between the laser beam 100 and 110 even in a standstill condition free from an angular velocity. Furthermore, this frequency is increased when a clockwise rotational angular velocity is applied to the gyroscope and decreased when a counterclockwise angular velocity is applied to the gyroscope.
FIG. 13A exemplifies changes of angular velocity applied to the gyroscope taking a positive side and a negative side of an angular velocity xcexa9=0 as clockwise and counterclockwise respectively. An example of angular velocity traced in a solid line 51 indicates that a clockwise angular velocity changes into a counterclockwise angular velocity with time lapse.
FIG. 13B which shows changes of a terminal voltage Vg of the gyroscope corresponding to the changes of the angular velocity indicated by 51 indicates that a changing frequency a beat voltage changes lower as the angular velocity changes due to interference with a laser beam as indicated by a curve 52.
FIG. 13C is a voltage waveform diagram of a rectangular wave 53 which is obtained by comparing the terminal voltage indicated by 52 with a level of Vref shown in FIG. 13B. Information of the angular velocity can be obtained by measuring, for example, time intervals of rising edges of a voltage of the rectangular wave 53 or a number of edges within a predetermined time.
FIG. 14 shows relationship between an angular velocity applied to the gyroscope and a beat voltage taking a beat frequency at an angular velocity 0 as fo. In FIG. 14, a maximum angular velocity within a necessary range for detection is denoted as xcexa9mx, a beat frequency at this angular velocity is denoted as fmx, a minimum angular velocity within the necessary range for detection is denoted as xcexa9mn and a beat frequency at this angular velocity is denoted as fmn. xcexa9o denotes an angular velocity at which the beat frequency is 0. A straight line 54 indicates that the beat frequency is enhanced as an angular velocity is increased in a clockwise direction and lowered as angular velocity is increased in a counterclockwise direction. Appliances which utilize built-in gyroscopes (for example, cameras, binoculars and navigators) have necessary ranges for detection of angular velocities and necessary detecting resolutions demanded dependently on characteristics of the appliances respectively. In a case where an angular velocity is detected with a gyroscope built in a still camera to prevent image blur, for example, it is sufficient, as already known, to detect the angular velocity with a resolution on the order of 0.1 degree/second within a range on the order of xe2x88x9220 to +20 degrees/second. In an example of the above described camera, xcexa9mx=+20 degrees/second and xcexa9mn=xe2x88x9220 degrees/second. Furthermore, there are a frequency characteristic and so on which are required dependently on characteristics of the appliances utilizing the built-in gyroscopes and in case of an image blur preventive system of the above described still camera, it is necessary to prevent a phase delay from exceeding xc2x115xc2x0 within a range of DC to 30 Hz in particular taking an overall frequency characteristic of the system as a bandwidth on the order of DC to 100 Hz. For configuring a image blur preventive system for a still camera which satisfies such a frequency characteristic, it is known that angular velocity due to blur should be detected at a period of at least approximately 1 msec. taking into account a time required for digital operation processings.
The ring laser gyroscope which indicates information of angular velocities as frequency changes of a terminal voltage is incapable of satisfying the frequency characteristic of the above described system when the frequency changes of the terminal voltage at time intervals of detecting the angular velocities. Specifically, it is necessary in a system which must detect angular velocities at intervals of 1 msec. that the ring laser gyroscope outputs a beat frequency higher than at least 1 kHz (at intervals shorter than 1 msec.) to a terminal voltage over an entire necessary range for detection of the angular velocities.
Even though it is conceivable to adopt a processor circuit which detects angular velocities as voltage changes converted from frequency changes caused due to angular velocity changes with an FV converter circuit, an analog circuit such as an FV converter circuit comprises parts such as a capacitor and a resistor which can hardly be integrated, and such a processor circuit is not suited for use from viewpoints of a disposing space and a manufacturing cost in a case where such an angular velocity detector is to be disposed in a compact electronic appliance such as a camera in particular.
Furthermore, manufacturing variations of individual gyroscopes and changes of gyroscope characteristics caused by changes of environments of use make it impossible to detect angular velocities accurately.
The present invention therefore has a problem to provide an angular velocity detector which is suited for integration and is capable of accurately detecting angular velocities.
In one aspect of this invention, an angular velocity detector comprises a ring laser including a tapered light waveguide path which partially has such an asymmetrical shape as to differentiate oscillating threshold values for laser beams propagating circumferentially in rotational directions opposed to each other, an optical gyroscope having a terminal which detects current changes, voltage changes of impedance changes of the ring laser, a measuring device which measures information of periods or frequencies of changes of a current, a voltage or impedance outputted from the terminal of the optical gyroscope, and a first output circuit which outputs information of periods or frequencies of the changes of the current, the voltage or the impedance outputted from the above described terminal in a condition where an angular velocity is not applied to the above described optical gyroscope, and is configured to calculate information of angular velocities by inputting a result measured with the above described measuring device and the information of the above described periods or frequencies outputted from the above described first output circuit, suited for integration and capable of maintaining a high accuracy.